Battery Voltage Drop

[Mike S]

I have (3) BigBattery Trex's on my battery bank being charge by two Solark 12K's at 59.6V absorption and 59.4 Float. I notice after the charge cycle is complete and there is no longer solar or some other form of charge my voltage drops from 59.4 to the 56's within a 10 minute time frame.

I have a 4th battery, currently back at BB for repairs, I only noticed this voltage drop after BB had me drop charge voltages to 57 for 10 days for a COV error on one of the batteries

Question is, is this normal? attached are screen shot examples of whats going on.

 

[sunshine_eggo]

LFP is not like LiPo. It's more like lead acid in terms of dropping from it's absorption voltage. Lead acid rapidly drops from 57.6V to 54V and will settle to around 52V in 24 hours. LFP is similar, but with a smaller magnitude and lower rate of settle after the initial drop.

Different cells, size, types, and brands behave differently.

Additionally, if your Solarks are ON, they are likely drawing from the batteries even with no AC loads, so they will pull the voltage down.

Any resting voltage above 54.4V is typically fully charged or 99%+. These cells can eventually settle notably lower.

I let Eve 280Ah cells sit fully charged for 5 months in an airconditioned space. Voltages dropped as low as 3.30V/cell (ranged 3.30-3.45V). All cells were at 98.5% or higher SoC.

Nothing in your charts is disturbing.

 

[byteharmony]

@sunshine_eggo thanks for helping with this. I'm learning about this drop just this morning as I'm building my German inspired powerwall:

100KWh DIY 18650 Powerwall in Germany

sorry, but the point of view of the picture above is not very good, but at the following picture below you can see the gap between roof and modules better. It's bigger than the roof overhang and shadow comes only when sun turns over to the west side. The balcony is exact east sided

diysolarforum.com

My 18650 cells came from batteryclearinghouse:

200 count of mixed 18650 cells in 100 assorted modem battery packs ($0.20 each!)

Hot deal is pun intended, these are mostly the older red Sanyo cells (aka "heaters"), these cells can get pretty warm/hot when charging and discharging so please use them at 1A or lower. These are not recommended for users new to the hobby. We have not tested these,or opened all the packs, there...

www.batteryclearinghouse.com

Most are red cells with amazing rubber and wiring in Motorola packaging (what is that rubber, does anyone know, I'd like to use that to hold cells together!!)
These cells all "Wake up" when I charge them and seem to hold a LOT of power.

Most of my experiance with Lithium comes from LIPO RC toy batteries. I charge them to 4.2v, play with them down to 3.6v, then charge them to 3.8v for storage. I've got some abused cells that lose voltage as time passes, they can fall below 3v if I don't give them a trickle from time to time.

Is this the same as what is happening with my 18650? Should I expect my powerwall to leak power?

Does the same thing happen to LFP?

I was thinking of building HUGE batteries for all the toys (electric lawn tractor, electric snowmobile, electric bike, electric boat, etc.) build them all at 48v and then keep them on my sol ark 14k in parallel so that should my house lose AC Grid power the Sol Ark can keep the house running with these batteries. I was hoping to have enough power to last 1.5 overnights. That way I'd never have to use a gas powered generator. The sun coming up on my 14k solar array would take care of charging them back up (48v * 275AMPS from the sol ark = 13kw, still juice left over to charge up!).

Will all these batteries be leaking lots of power losing $?

 

[sunshine_eggo]

3.7V chemistry does not experience resting voltage settling like LFP. If you are experiencing individual cell voltage settling, that's an indication of excessive self-discharge.

Take 20 cells. Charge them to 4.20V hold until 0A . Stop charge and set them aside as individual cells (do not let them sit in parallel). Check them in a week. The bad ones will stand out.

With LFP, the voltage settling is not correlated to energy loss. Example:

Cell 1 charged to 3.65V to 0A. Rest 30 minutes. Cell voltage might settle to 3.55V
Cell 2 charge to 3.65V to 0A. Rest 24 hours. Cell voltage might settle to 3.45V

You will not be able to measure a stored energy difference between the two on a discharge test.

 

[byteharmony]

3.7V chemistry does not experience resting voltage settling like LFP. If you are experiencing individual cell voltage settling, that's an indication of excessive self-discharge.

Take 20 cells. Charge them to 4.20V hold until 0A . Stop charge and set them aside as individual cells (do not let them sit in parallel). Check them in a week. The bad ones will stand out.

With LFP, the voltage settling is not correlated to energy loss. Example:

Cell 1 charged to 3.65V to 0A. Rest 30 minutes. Cell voltage might settle to 3.55V
Cell 2 charge to 3.65V to 0A. Rest 24 hours. Cell voltage might settle to 3.45V

You will not be able to measure a stored energy difference between the two on a discharge test.

LFP: Voltage settling is loss? When it gets to 0A is it actually just a smaller current draw, say 1mA, that is the voltage loss that will exist in a good cell. Just in the same way that a lead acid based battery charged to 14.2v will continue to accept power because it will rest at 12.6v. Even if charged to 12.6v a good lead acid battery will still accept power because it's not "perfectly stored power".

RE 3.7V: "The bad ones will stand out... I've seen all cells fade in voltage, some more than others.

I'm now thinking every battery chemistry has a level of power loss that takes as energy is stored, discharged and charged. They are not perfect theoretical containers of energy.

Some are more efficient at the above processes.

Some are more energy dense.

Some have better temperature resistance.

Some have better charge discharge current capability.

Some have flatter discharge curves.

Some have higher cost per unit of energy.

Some have a higher cycle count.

Most, if not all, should not be combined to form larger batteries.

What makes sense for me I used to think I knew, the more I learn the less confidant I feel.

I was thinking a mix of 14s 3.7v cells and 16s 3.6v cells.

I was thinking a mix of DIY power wall with recycled vehicle batteries. This gives me experience and is fun.

I was thinking a mix of fixed mounted power and mobile power. As the mobile power cycle count grows, replace it with new mobile power cells and recycle the replaced cells as fixed mount cells.

If I add cell voltage loss to these ideas, bad cells may become expensive from the perspective of power loss as the battery gets bigger. In other words, the amps required to maintain the charged state voltage I choose grows as the battery gets larger.

What's true above, what's not?